I'd actually look at aftermarket speakers before a sub, but that's me.
Technically speaking, the subwoofer plays a very tiny range of the audio spectrum. Unless you're just looking for boom, the sub should really take a back seat to the build up of the rest of the system.
Why 2000 watts? You want people 2 blocks away to hear your car? You'll be suprised how little power you need when using the correct sub to get good output. There are options that run off only a few hundred watts. Again, if you want boom, you're looking at multiple subs, 2, 3, 4, and a resulting large amount of power.
When considering audio upgrades, think of the system as a whole. Figure out what your goals are and what areas you'd like to improve. If you just want some serious thumping bass and that is your design goal, all you need are several subs and a lot of wattage. However, if you're looking for good overall sound and a system that's well balance, you're going to start with a good set of full ranged speakers powered by an appropriate amp. Then you accent, again appropriately, with a subwoofer to fill the bottom end.
You really should plan and find out what you really need to achieve your goals.
I will note, if your car is completely stock and stock is all you're used to, almost anything aftermarket will most likely impress you. Even with subwoofers, a single, entry-level 10" or 12" sub powered by less than 500 watts will impress.
I write this as I hope I can get you thinking more about car audio than just boom. I know a lot of kids start out this way, stock system and pounding subs. I don't know why they do this other than attention. It doesn't sound well, just annoying. I start by upgrading what will provide the greatest overall gain and benifit.
My build order is: 1) Speakers - either entry level but good sounding coaxials to run off the HU as a level 1 build, would be replaced in the future and considered disposable, a cheap upgrade ...or... a set of mid to high end components with the intent of being exactly what I want and will never be upgraded further. As well, they would either be amped immediately or in the near future when funds become available 2) Amp - amp to power full range speakers, either bought immediately with the speakers or on hold until funds are available 3) HU - Again, cheap or good. A cheap one may get you buy for a year or two, but a good one would last you a very long time(i.e. offers all the features you want and will ever want in the future) 4) Subwoofer - I really do place this last. I perfer to have the rest of the sytem up to par first. This is just added to fill in the bottom end of the frequency spectrum. An amp would obviously be purchased for the sub.
This is just the way I look at a car audio build. If I have the money, it all comes together at once. If not, I follow that order, generally buying exactly what I want to achieve my end result.
Matt, just want to give you props for taking the time to write that up. Those are pretty much my exact sentiments on car audio and anybody who reads this would do well to heed your advice.
Joe, if you can crank up your speakers now and they don't blow, then they won't automatically blow with the sub installed as long as you're not keeping them cranked at all times, but I'm willing to bet that when you get them to about 3/4 volume now they start to distort so they'll obviously do the same once you get the sub in and after prolonged use, they're bound to give up the fight. The "2000 Watt max" doesn't really tell us much because the actual output will depend on your amp and the RMS wattage of the sub, but like Matt said if you care at all about sound quality you'll need new speakers and most likely an amp for them to keep up with your sub and have any semblance of sound quality. But, if you're just looking for boom to irritate the neighborhood and set off nearby car alarms, then there's no reason to bother with speaker upgrades.